3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
Examples of techniques employed in the 3GPP LTE-A include carrier aggregation.
The carrier aggregation uses a plurality of component carriers. The component carrier is defined with a center frequency and a bandwidth. One downlink component carrier or a pair of an uplink component carrier and a downlink component carrier is mapped to one cell. When a user equipment receives a service by using a plurality of downlink component carriers, it can be said that the user equipment receives the service from a plurality of serving cells. That is, the plurality of serving cells provides a user equipment with various services.
Meanwhile, a packet data convergence protocol (PDCP) is used for functions of user data delivery, header compression, and ciphering in a user plane and used for functions of control-plane data delivery and ciphering/integrity protection in the control plane.
And, a PDCP status report is used to inform a transmitting side about a status of a reception buffer for a PDCP entity of a receiving side.